Why did it take him that long? And if he "returns" to Middle Earth, where did he spend the two and a half millenias in between?

Furthermore: Where did the additional strength come from? Obviously not from the One Ring, since it was lost or (as far as Sauron knew) destroyed. If this strength has been with him all the time, why did he not draw from it when Isildur cut the Ring from his Hand?

6 Answers
6

The reason it took so long for Sauron to "return" (considering he never really left Middle Earth) was because he had put most of his power, strength and will into The One Ring. This meant that even though his physical form was destroyed he would still exist.

The Ring of Power was created by Sauron. They are one. Sauron survived
because the Ring did.

Even when his physical body is destroyed it doesn't mean he loses all his power. This is demonstrated when Sauron is drowned by Eru after destroying Númenor; he was able to move The One Ring even without a body. Then his body was destroyed a second time as well.

Despite his defeat, Sauron was not vanquished permanently. Though
greatly weakened, and in non-corporeal form, he still existed, due to
pouring most of his native power, strength, and will into the One
Ring. Thus, as long as it existed, he could never be truly defeated,
and during the first thousand years of the Third Age, he lay in
hiding, slowly recovering his strength until he was once again able to
create a body for himself.

He had already gained enough power around the first millennium of the third age and was disguised as the Necromancer at the Dol Goldur. Gandalf after failing a first attempt at finding out if the Dark Sorcerer was Sauron in TA 2063, successfully discovered that Sauron had returned during a second intrusion at Dol Goldur in TA 2850.

Gandalf the Grey made a second intrusion into Dol Guldur in TA 2850,
and finally discovered that the Necromancer was indeed Sauron.
Eventually, the White Council put forth their might and drove Sauron
from Dol Guldur permanently in TA 2941. Without the Ring in his
possession, Sauron could draw on only the smallest fraction of its
strength, so that his enemies were able to drive him from Dol Guldur
with relative ease.

Essentially the problem with Sauron is that he put most of his essence into The One Ring, he still had powers that rivaled most mortals in Middle Earth while The One Ring existed. Plus he was a Maia which was one of the Ainur.

The below quote is from http://oakroadsystems.com/genl/ringfaq.htm#Q1-ReturnFromNumenor
It speaks about how Sauron was able to move the One Ring after his physical body was destroyed at Numenor the first time when Eru directly intervened and Númenor was drowned under the sea. L#211 is one of the Letters from Tolkien as described in the bottom of that website.

Tolkien simply brushes the difficulty aside: “Though reduced to ‘a
spirit of hatred borne on a dark wind’, I do not think one need boggle
at this spirit carrying off the One Ring” back to Middle-earth after
the drowning of Númenor. [L #211 (280)] The solution is that Sauron
was a Maia, one of the original Ainur. The Ainur had the power to
manipulate the physical world by pure effort of will; that’s how they
finished shaping Arda after entering into it in its “raw” state after
creation. We know that Sauron in particular had this ability because
he created a body for himself several times.

It's stated by Gandalf at his "final death" that the power he had originally had was gone now.

"If [the Ring] it is destroyed, then he will fall, and his fall will
be so low that none can foresee his arising ever again. For he will
lose the best part of the strength that was native to him in his
beginning, and all that was made or begun with that power will
crumble, and he will be maimed for ever, becoming a mere spirit of
malice that gnaws itself in the shadows, but cannot again grow or take
shape. And so a great evil of this world will be removed."

With respect to how easy it was to drive Sauron from Dol Guldur, don't forget that the Nazgul were in Mordor as early as 1980, presumably preparing it for Sauron. I'm sure that by 2942, when he returned there, he was more than ready.
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Matt GuttingJun 12 '14 at 16:21

If he was "that ready" he would have declared war on Middle Earth before fleeing the White Council. And are you talking about Second Age Or Third Age 1980? Because according to the appendices of Return of the King they first appeared in SA 2251. Then dispersed at around the first overthrow of Sauron, and re-emerged as around 1300's Third Age.
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DoctorWho22Jun 12 '14 at 17:42

Sorry - Third Age 1980. And I just realized I didn't fully explain myself in the comments - I should have said "...when he returned there [to Mordor], he was more than ready [to move back]."
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Matt GuttingJun 12 '14 at 17:44

As far as preparing Mordor for Sauron I don't think that was the case, they captured the city Minas Ithil and it was renamed Minas Morgul becoming the Nazgul stronghold. Other than that I remember at least up to three Nazgul protecting Don Guldur at one point too.
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DoctorWho22Jun 12 '14 at 17:49

The idea that Sauron could only draw on a portion of his strength without the Ring is false: "But even if he did not wear it, that power existed and was in 'rapport' with himself: he was not 'diminished'."
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ShamshielJun 13 '14 at 0:49

Well, we don't know exactly how long it took Sauron to re-assume physical form - we know how long it took him to make his presence known. But first of all, the One Ring has nothing to do with Sauron being able to come back quickly or not. From Tolkien's letters

But to achieve this he had been obliged to let a great part of his own inherent power (a frequent and very significant motive in myth and fairy-story) pass into the One Ring. While he wore it, his power on earth was actually enhanced. But even if he did not wear it, that power existed and was in ‘rapport’ with himself: he was not ‘diminished’.

Sauron was not in any way diminished by not wearing the Ring. His powers were simply not enhanced. We are never told, nor is it implied, that one of the powers the Ring enhances is shapeshifting, or the ability to form a body. Sauron forged the One Ring in order to accomplish the following:

This was the essential weakness he had introduced into his situation in his effort (largely unsuccessful) to enslave the Elves, and in his desire to establish a control over the minds and wills of his servants. There was another weakness: if the One Ring was actually unmade, annihilated, then its power would be dissolved, Sauron's own being would be diminished to vanishing point, and he would be reduced to a shadow, a mere memory of malicious will.

The One Ring was to control the wielders of the Three Rings and dominate the minds and wills of his servants. It enhanced his native capability to terrify and dominate minds, which he had posssessed for millennia, documented in The Silmarillion. But without possession of the Ring, he was not diminished, he was just plain old Sauron again.

The more likely explanation is simply that an Ainur's restorative powers are not infinite. When they are injured or killed, those injuries exist on more than a physical level, even though the assumption of material form is supposed to be like raiment. We have a lot of evidence for this. For example, even after he again took material form and announced himself, he was still missing a finger. From "The Black Gate Closed" LotR chapter:

‘Yes, He has only four on the Black Hand, but they are enough,’ said Gollum shuddering. ‘And He hated Isildur's city.’

After he drowned in Numenor, it took him some time to regain a physical form, and he was not able to appear fair anymore.

From Letters:

Sauron was, of course, ‘confounded’ by the disaster, and diminished (having expended enormous energy in the corruption of Númenor). He needed time for his own bodily rehabilitation, and for gaining control over his former subjects. He was attacked by Gil-galad and Elendil before his new domination was fully established.

From Akallabeth:

But Sauron was not of mortal flesh, and though he was robbed now of that shape in which he had wrought so great an evil, so that he could never again appear fair to the eyes of Men, yet his spirit arose out of the deep and passed as a shadow and a black wind over the sea, and came back to Middle-earth and to Mordor that was his home. There he took up again his great Ring in Barad-dûr, and dwelt there, dark and silent, until he wrought himself a new guise, an image of malice and hatred made visible; and the Eye of Sauron the Terrible few could endure.

In The Silmarillion, Sauron's injuries clearly remain regardless of the form he takes:

But no wizardry nor spell, neither fang nor venom, nor devil's art nor beast-strength, could overthrow Huan of Valinor; and he took his foe by the throat and pinned him down. Then Sauron shifted shape, from wolf to serpent, and from monster to his own accustomed form; but he could not elude the grip of Huan without forsaking his body utterly. Ere his foul spirit left its dark house, Lúthien came to him, and said that he should be stripped of his raiment of flesh, and his ghost be sent quaking back to Morgoth; and she said: ‘There everlastingly thy naked self shall endure the torment of his scorn, pierced by his eyes, unless thou yield to me the mastery of thy tower.’

Then Sauron yielded himself, and Lúthien took the mastery of the isle and all that was there; and Huan released him. And immediately he took the form of a vampire, great as a dark cloud across the moon, and he fled, dripping blood from his throat upon the trees, and came to Taur-nu-Fuin, and dwelt there, filling it with horror.

It seems clear that, over thousands of years, as Sauron spent his strength in evil and the domination of others, his ability to change form diminished. This is exactly the same thing that happened to his master Morgoth, except all of Arda was Morgoth's Ring.

Morgoth was eventually restricted to one form, was unable to heal his wounds, and is spending thousands and thousands of years regathering his strength so he can take form (the Valar beheaded him and expelled him from the world) to start the Last Battle. But it was a gradual process, just as it seems to have been with Sauron, that got worse over time.

Alas, I recall an even clearer statement by Tolkien that spending your time dominating others and dominating others weakens Ainur and affects their physical form, but I can't recall where it is. It doesn't seem to be in Letters.

Nevertheless, it seems clear it took Sauron so long to recover because he had been beaten so badly - Elendil and Gil-Galad "overthrew" him - and because he had been beaten so often, and still hadn't even recovered from Numenor. We can certainly imagine Sauron expended enormous energy in inspiring the troops to fight off the Last Alliance and the Siege.

EDIT: This is almost certainly what I was thinking of, from The Flight of the Noldor.

For now, more than in the days of Utumno ere his pride was humbled, his hatred devoured him, and in the domination of his servants and the inspiring of them with lust of evil he spent his spirit.

This was Morgoth, but Sauron did the same. Sauron's personal strength declined with time because of this.

If the ring had been destroyed, as Sauron knew, he would have been destroyed along with it. Well, not exactly destroyed. Gandalf describes the fate of Sauron at the destruction of the Ring this way:

"He will lose the best part of the strength that was native to him in his beginning, and all that was made or begun with that power will crumble, and he will be maimed for ever, becoming a mere spirit of malice that gnaws itself in the shadows, but cannot again grow or take shape."

Obviously this didn't happen to Sauron; but he was separated from most of his power and presumably it became more difficult for him to do things.

Now it looks like your interpretation of the wiki entry is incorrect, as is its phrasing. The entry states:

2460 Sauron returns to Middle-earth; establishes himself in Dol Guldur in southern Mirkwood.

So where was he before he returned? Well, the entry for 2063 gives us:

Gandalf goes to Dol Guldur. Sauron retreats and hides in the East.

So it looks as if Sauron was alive and well and established in Dol Guldur by then. Indeed, a thousand years earlier, we get

1050 Hyarmendacil conquers the Harad. Gondor reaches the height of its power. About this time a shadow falls on Greenwood, and men begin to call it Mirkwood.
1100 The Wise (the Istari and the chief Eldar) discover that an evil power has made a stronghold at Dol Guldur [in the south of Mirkwood]. It is thought to be one of the Nazgûl.

(emphasis added)

So rather than 2,460 years, I'd say 1,000 years is more accurate. Given that the chronology of the Second Age tells us that it was about 500 years between the end of the First Age and the time Sauron began to become active, I'd say that 1,000 years for a significantly weakened Sauron is not surprising.

As far as "where was he" during that millennium, I'd say that he was probably in Middle-earth, but unable to do much. He hadn't lost any power, but it appears that, separated from the Ring, he had lost much of his ability to use his power. (This was the mistake that Morgoth made: pouring his power into something without realizing, or perhaps simply without caring, that his ability to use it as he wished would thereby be diminished.) His minions had not deserted him, but without a physical body (which I believe he must have regenerated after some time) it would have been difficult to command most of them.

He didn't fully announce his return until TA 2951 when he openly declared war against Middle Earth, it should be said he was at the height of his power in non corporeal form at that point.
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DoctorWho22Jun 12 '14 at 15:50

And technically speaking Sauron never left Middle Earth... He was just in a weak non corporeal form.
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DoctorWho22Jun 12 '14 at 15:52

I'm not sure when (or even whether) Sauron assumed a body again. We're not told that he did. (Unless I've missed something in the appendices or the Silmarillion.
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Matt GuttingJun 12 '14 at 16:01

There's this quote as well : "True, alas, is our guess. This is not one of the Úlairi, as many have long supposed. It is Sauron himself who has taken shape again and now grows apace; and he is gathering again all the Rings to his hand; and he seeks ever for news of the One, and of the Heirs of Isildur, if they live still on earth." The Silmarillion, Of the rings of power and the third age
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DoctorWho22Jun 12 '14 at 18:37

Consider "To regain strength" has more than one meaning
"To return to full health" to use a DnD metaphor, he'd lost a huge number of health points (say from many thousands to 1), he needed to recuperate so he would be strong enough to stand up to others (the head bad guy tends to be the head guy because he is stronger and meaner than the rest).

It also means "to rally forces", he basically had to go back to scratch and re-build an empire that could physically match the combined might of Elves, Men and Dwarves, not something he can do over night.

Consider also that Tolkiens "System of magic" isn't about wizards throwing energies about like Ghostbusters firing proton streams at ghosts, but more like the "powers" that philosophers of old had, which comes from understanding the world and the forces of nature. Tolkien "magic" generally worked slowly.

It is implied more than once that if he had attained the ring he would have become incredibly powerful very quickly. Instant restoration of health points, and given his key ability to sway the minds of others, very quickly build his evil empire.

All in all it seems perfectly reasonable to me that it could take millennia for a hugely weakened maleficent spirit to return to it's previous status as a powerful being with an empire under his control.

It may not have taken as long as you say. The realm of Angmar was founded in TA 1300 and was run by the Nazgul King. As the Nazguls were under the influence of Sauron at all other times, it is reasonable to assume they were then as well.

What was Angmar up to? It fought and finally destroyed the Northern Kingdom of Arnor piece by piece. In the second age, Elves and Men united brought down Sauron. This eliminated half of the Kingdoms of the Dunedain from participating in any future action. Within a few years, Angmar was brought down and the Witch King fled to Mordor in TA 1975.

And guess what happens almost at once? The Witch King takes Minas Ithil, renaming it Minas Morgul. He started challenging the childless King of Gondor to combat, which was accepted in TA 2050 and the last King was lost and never returned.

So it appears that a good portion of this "lost time" was in fact a premeditated plan to destroy and weaken the Kingdoms of Men.

After the battle with Gilgalad and Elendil, Sauron took a long while to re-build, longer than he had done after the Downfall of Númenor (I suppose because each building-up used up some of the inherent energy of the spirit, which might be called the 'will' or the effective link between the indestructible mind and being and the realization of its imagination). The impossibility of re-building after the destruction of the Ring, is sufficiently clear 'mythologically' in the present book.

So, each time Sauron needs to "re-build" he uses up some of his strength, and it therefore takes him longer the next time.